From: John Larkin
Subject: Re: My flyback experiment is on the web
Date: Mon, 14 Oct 2002 08:05:32 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Sun, 13 Oct 2002 23:40:41 GMT, Chris Carlen
>Here is the URL to my web page showing the results of my most recent
>experiments with a flyback converter. I still haven't gotten a PWM chip
>hooked up yet, as my first goal was to buy some inductors that look like
>they are designed for switching supplies, and see if they'd work better
>than the hand wound transformer I made from a PC power supply core.
>Actually, the inductor seems a bit worse :-(
>All the details are on the web page.
>My main question is why does the inductor get so warm? I figured I
>wasn't really pushing it too hard, so it shouldn't loose much power.
>but it seems most of my losses are occurring in the inductor. I was
>hoping to avoid complex topologies such as a two transistor flyback,
>which requires a transformer, but lends itself to easily shunting the
>remaining core energy back to the input rail. Can that be done with the
>simple flyback that I am working with? I figured I could get 75% or
>better, so I'm not really far off. But I don't like heating the
>inductor, and I want to push up the power another 25% from what I'm
>testing at before I finish.
>I have a smaller inductor, and a larger one too, but all of the toroidal
>type, designed for 150kHz switchers.
>Comments appreciated on what steps I might take to make this simple
>circuit move in the direction of something worth committing to FR-4.
>I will put more on the web page when I get to testing out the PWM chips,
is the torroid core yellow? Sort of looks like that in the photo. If
so, it's probably a dreadful Micrometals powdered-iron core.
It also looks, from the waveforms, like you may have a lot of fet
drain capacitance... I'm too lazy to look it up. That will waste
energy, specifically 1/2 * Cd * Vp^2 every cycle. This is a, maybe
*the* serious problem in high-step-ratio converters like this; at some
point, all the energy charges the fet and vout becomes limited to the
flyback peak. Then you need a transformer.